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Gert - I'd love to get away with 3/8" hose and couplings, but the plumbing has to be sized for the pump CFM. In my case about 200 CFM, where 2" is what the engineering charts point to for free air flow. ...

-B

Wow, that is a lot of volume for pods and the skinny hose surely won't do. In my case I have only a high vacuum GAST pump that does maybe 4 cfm but I have used it successfully for custom shaped pods up to 15" x 20" or maybe double that size when 2 pumps are used.

I have modified my 36"x48" 80/20 table (consisting of 3-slot 4515 extrusions) that a few of the middle slots are closed (thin aluminum flashing taped on) and act as the vacuum conduit. I can punch a hole anywhere along the slot and only need a re-usable non-stick gasket (cut from foam-type drawer liner) to hold down a board for one-sided work, no real "pod" needed. I even hold down the indexer and tailstock or the machine vise that way when used. But that would not work as well for your high volume vac system.

When using any aluminum table on a steel frame, keep in mind the thermal expansion of aluminum is twice the expansion of steel and for an 8' long machine that can become substantial. It can be 10-15/1000" difference for a 20 deg F change which does not sound much but could bow the table when not allowed to move relative to each other (e.g. mounted on standoffs).

Just a thought,you have a lot of wood products right now, (base, plenum, spoil) that's a lot room for movement. Instead of using 8020, why not use a sheet of 3/4" 6061 Aluminum plate? With the 8020 you have a lot of smaller pieces that have to be aligned and secured to each other or they can move independent of each other. A sheet of 6061 weighs around 514 lbs. A bit heaver than 8020, yes, but a lot more stable. A few extra cross members will handle the weight. A heaver machine is a more stable machine. The expansion coefficient is minimal. You could machine your grid right in to it and use gasketing or use a structural bonder and attach another plenum board.
With regards to the MDF shrinking, could it be with a 13 hour run, the moisture content is going down in the board with that much air flow going through it? That time of the year, dry, dry,dry. With the big guys running heavy iron, there changing there spoil boards so frequently they don't have a problem with it shrinking.
Double refined MDF. Try Ranger Board by West Fraser. Awesome product, finishes out after machining like it has been sanded to 150. Also no extra edge sealing, the edges absorb the same as the flats.

I was considering sheet AL (probably 1/2" is more than adequate, but $900/sheet @ 4x8' x 3 = $2700) and phenolic sheet as well; but to cover a 5x16' area they're both more expensive and have less utility than the 8020, because both lack t-slots. I'd like to bump up to the 1.5" stuff (1530/1545), but that brings the grand total closer to $4k for the job. Plus, a 1/2" or AL sheet is cumbersome to handle even with a front loader and cherry picker. It would also be dicey putting MDF on top for a bleeder...

Initial testing of a piece of 1030 shows that it's pretty darn stout between the crossmembers and I'll never put enough weight on it to have it deflect any measurable amount. I don't think expansion and contraction is going to be much of an issue. There's still no getting around doing a flattening routine to get things flat to the gantry/spindle from time to time. If I sense that things are too up or down, bolts holding the substrate to the 8020 can be loosened to allow it to relax and then tightened down again. As I said, the DT tool has proven to be excellent for flatness...granted this is a much larger area, but I think it's way better than relying on wood with temp/humidity swings.

I'm leaning towards replacing the 2x2x1/4" angle iron crossmembers with 2x3x1/4" wall steel tubing...It will provide twice the support and deflection resistance according to deflection calculations.

Yes, it's a lot of work getting them properly secured, aligned and flat - BUT - I only have to do it once. Plus, I have robots to drill holes for me exactly where I want them. There's only going to be about 860 of them by my calculation...

Thanks for the tip on the Ranger Board - I'll check it out.

Originally Posted by Burkhardt

Wow, that is a lot of volume for pods and the skinny hose surely won't do. In my case I have only a high vacuum GAST pump that does maybe 4 cfm but I have used it successfully for custom shaped pods up to 15" x 20" or maybe double that size when 2 pumps are used.

I have modified my 36"x48" 80/20 table (consisting of 3-slot 4515 extrusions) that a few of the middle slots are closed (thin aluminum flashing taped on) and act as the vacuum conduit. I can punch a hole anywhere along the slot and only need a re-usable non-stick gasket (cut from foam-type drawer liner) to hold down a board for one-sided work, no real "pod" needed. I even hold down the indexer and tailstock or the machine vise that way when used. But that would not work as well for your high volume vac system.

When using any aluminum table on a steel frame, keep in mind the thermal expansion of aluminum is twice the expansion of steel and for an 8' long machine that can become substantial. It can be 10-15/1000" difference for a 20 deg F change which does not sound much but could bow the table when not allowed to move relative to each other (e.g. mounted on standoffs).

Gert,
I also have a couple of little Gast pumps and use the smaller tubing with them. I learned early on to err on the side of bigger is better when it comes to vac tubing because I remember not being able to make a seal even with an 11-gal surge tank because the tubing was too small...'like sucking through a straw' - quite literally! It certainly did cross my mind to exploit the internal cavities of the extrusions for vac plumbing...but aside from needing multiple feeds to make up for the smaller equivalent tubing diameter, there was also the issue of sealing both ends of the extrusions air tight. That means drill/tap/gasket and bolt at least 40 caps...I think for me, on-board PVC pods etc with them bolted to the 8020 would be fine. In fact, it is very rare for me to use the small vacs on the big machine because the bigger vacs are available right there.

Most of the time I'll just make a 'direct air' pod, puck or fixture out of whatever scrap I have laying around. I made one last week, printed out an adapter and plugged in some 1.5" corrugated hose...The hose held up under 25 Hg" - so I am considering using some of it for the 'quick release' setup when I want to remove the vacuum grids and just machine on the 8020. I reverse engineered a push to connect fitting and scaled it up to fit a 2" PVC pipe...the problem is that it doesn't scale well - looks goofy(!) - and while it seemed like a good idea on paper, it's too wonky to use in real life. Then I was thinking about something more compact - why not a compression fitting?...then again, why not just plug a 2-2.5" ShopVac hose in or out of the zone and be done with it?! So I am thinking in this direction now. They have nice hose end fittings/cuffs for the corrugated pool/car wash hose, it won't crush down and leakage doesn't seem to be a problem. I'd just adapt from the existing PVC manifold or run to a flex hose and be able to plug/unplug as needed.

In terms of expansion/contraction - I'm going to see how it goes. My initial thought is to slot the thru holes to both deal with the possibility of piece to piece stack up errors and possibly give the material a place to go if it does expand. This is a tightrope between getting the proper torque to hold things down and allowing it to move. I can't say with any certainty that the MDO doesn't buckle/bow under weather changes - because I just don't have the instrumentation to pick that up. I will say, visually I can't tell - even with a straight edge and dial indicator. Last week for the heck of it, the shop was around 70F and I pre-loaded the dial indicator against the spoilboard. When I came in the next morning there was zero change on the indicator at 46F. Not very scientific, and kind of a novelty test, but I was expecting a change of at least .005".

Attached are pics of the reverse engineered push to connect and how HUGE it got (like 4.5" OD) - and a jig I slapped together last week out of 3/4 & 1/4" MDF - no gasket required. The hose feeds in from the bottom off the table. To secure the jig to the machine, I just put like 10 dollups of wood glue behind the jig to the spoilboard and weighted it down. It came off pretty easily when done...but did take a few pocks of material with it when I pried it up.

For my clamping portion of my table I bought some extrusion from Hubbard CNC they sell on Ebay. If I had it to do again I would make my own clamping table from 1/2" x 4" 6061 bar stock by just cutting a rabbit out of the corner and placing them spaced so they make a T slot when installed. My extrusion fit 5/16" bolts if I were to make my own I would make it fit a 5/8" slot that way you can buy ready made clamping kits for milling machines.

Mike,
I certainly did consider doing exactly what you show. The problem is, 1/2" AL 6061 would actually cost a little less than the 8020 but still have to be machined, drilled and the steel drilled to accept all those pieces. It's definitely a great idea, but I think the 8020 is the way to go for my setup.

Saw the Hubbard stuff as well as the Techno stuff and other offerings on Ebay et al. All the 20mm stuff seemed too flimsy and I think they can only take a 5mm fastener - so that rules that out. I think 1/4-20 is the smallest I'd want to go.

I'm looking forward to popping my Kurt or Grizzly milling vices on the table. I'll have to make an adapter plate for quick registration, but it's great to be able to really crank down on the metal stuff when machining...Plus align it with an indicator and life is good. I had one on the DT machine when doing some cutting on an awkward to hold aluminum casting.

Got me thinking now Mike...I'm going to have to crunch numbers again and see if AL bar would really make sense for my work...

I remade my table plenum last year with 4 2' X 4' X 3/4" pieces of HDPE. I used four pieces so I didn't have to deal with expansion over the whole 4 X 8. This replaced a mdf plenum and the original hassle of sealing it still leaves a bad taste and then even after sealing it it was still barely sufficient compared to the plastic. The top cost, I think about $200.

I know that some have criticized hope as plenum over the years but, although I don't remember the numbers, the expansion over a 20deg temperature change was not bad and was accounted for by drilling larger mounting hole towards the positive x and y directions. The plenum is much more efficient and to this point the table has stayed fairly flat. My shop is air conditioned and heated so there is rarely a large temperature change.

I would add to the criticism of HDPE, even if it works in your air conditioned environment. Its rigidity is about 2% of steel and 6% that of aluminum so it needs a very good structural support and the thermal expansion is about 20 times that of steel.

Another thing comes to mind for any extruded material, be it plastic or aluminum that it has residual stresses from production and will deform from flat when machined, like skimming the surface or cutting slots or rabbets. There is an aluminum grade MIC6 that is stress relieved and will not deform much if machining is required.

I just replaced the mdf plenum and kept the two 3/4” birch ply sub base so that the hdpe structural concerns are not really an issue. The thermal expansion being 20 times that of steel doesn’t really matter. What matters is that the expansion is accounted for. The expansion on a 2 X 4 piece is still pretty small - at least by woodworking standards and this is dealt with in the mounting holes.

I use a Fein not a large blower like Brady and so I was not dealing with the same issues and trying to solve
the same problems. The suction improved immensely. After using this for a year, if I were going to redo it, I would do the same thing. The table is still pretty flat and when I need vacuum it’s better than the shellacked mdf. Best $200 to $300 I spent last year.

Any solution is a compromise , otherwise the choice would be easy and obvious. I do different things and have different requirements than Brady (and probably most SB'ers) and am not suggesting this as a solution for him or anyone else but it works well for me. I used an mdf plenum for 15 years and had plotted a replacement for years when I went with the hdpe. After a year's experience I cannot remember once lovingly longing for the mdf or encountering one time when I regretted not having an mdf plenum.

Given my stage in life, I measure return on investment in weeks and months - not years. Phenolic and 80/20 seemed like awesome choices when I was looking but I was not willing to spend $1000 plus. Having followed Brady's posts and having had a couple of conversations over the years with him, I have enormous respect and admiration for his work, his impressive knowledge and his sharing — I never skip a Brady post. His requirements are different from mine.

My only point is that using a "sealed" mdf plenum is akin to polishing a cow patty — it may look better but ...

HDPE - been there done that. In my shop it moved all over the place with temp & humidity swings. I couldn't get the fastener tension right to keep it from curling. Too loose & it moved around...too tight and it buckled. I've also serviced machines where they had HDPE and it moved around enough where the spoilboard was like a rainbow - all bowed up, with chips and swarf between it and the support board at the frequency of where the screws were holding it down. Now this machine was in Miami - and temp/humidity swings are more extreme than most of our shops...plus how dilbert do you need to be to not notice your table getting all jacked up like that? <sigh>

In the past I've used composite decking (Depot Veranda because it was solid...don't think so anymore) and after running it through the planer to flatten off the texture, it was pretty good for making long pucks. Not sure I'd want to do an entire surface with it, but for hard wood vacuum fixturing it was pretty good.

If I do wind up making a plenum out of a non-wood material, first choice will be aluminum plate and 2nd choice phenolic. Even PVC/Sintra/Komatex moved around a little more than I wanted it to on the BT48.

I say if it works for you - great - keep doin' it. I don't want the added expense of heating/cooling my shop when I am not in there - so I have to bite the bullet and figure out what works best in my situation. We shall see my friends!